Abstract

As humans continue to explore the last uncharted regions of the planet, discoveries of previously unknown species of large vertebrates have become infrequent. Here, we report on the discovery of a spectacular new species of giant, secretive, frugivorous, forest monitor lizard (Genus: Varanus) from the forests of the northern Philippines. Using data from morphology and mitochondrial and nuclear DNA sequences, we demonstrate the taxonomic distinctiveness of this new 2 m long species and provide insight into its historical biogeography and systematic affinities. Our molecular phylogenetic analyses indicate that the new species is closely related to Varanus olivaceus (from southern Luzon and nearby islands), but it differs from this and other varanids with respect to characteristics of scalation, colour pattern, body size, anatomy of the reproductive organs and genetic divergence. The new species appears to be restricted to forests of the central and northern Sierra Madre mountain range; it is separated from the range of V. olivaceus by a more than 150 km stretch that includes at least three low-elevation river valley barriers to dispersal. This discovery identifies a seldom-perceived biogeographic boundary and emphasizes the need for continued biodiversity research in the megadiverse conservation hotspot of the Philippines. It is anticipated that the new species will serve as an important flagship species for conservation efforts aimed at preserving the remaining forests of northern Luzon.

1. Introduction

Although the description of biodiversity is a continual process, discovery of previously undocumented, large-bodied and conspicuous new species of vertebrates is now a rare occurrence. As human populations increase, forested regions of the planet are charted, and large intact tracks of forest continue to shrink or become fragmented. With increasing exploration, fewer large-bodied vertebrates remain to be discovered. Exceptions to the asymptotic decline in the discovery of large new species of vertebrates include the discovery of the Vietnamese forest bovid Pseudoryx nghetinhensis (Dung et al. 1993) and the discovery of a new African primate (Rungwecebus kipunji; Davenport et al. 2006).

Monitor lizards occur throughout the Old World tropics (Bennett 1998; Pianka et al. 2004). They vary greatly in habitat (arid to tropical) and body size (V. brevicauda: less than 25 cm, 8–17 g versus V. komodoensis: greater than 3 m, 70–90 kg) and include the most massive lizards in the world (Komodo dragons). Fewer than 10 new species of monitor lizard have been described in recent years, and most have been small-bodied forms (Harvey & Barker 1998; Pianka et al. 2004; Ziegler et al. 2007). Philippine lizards of the family Varanidae include five species (Taylor 1922; Gaulke 1992): the water monitors (Varanus cumingi, V. marmoratus, and V. nuchalis), and the arboreal, frugivorous monitors (Varanus olivaceus and V. mabitang). Varanus olivaceus and V. mabitang, the world's only two frugivorous species, are rare and difficult to study (Auffenberg 1988; Gaulke et al. 2005). Shy and reclusive, V. olivaceus is known from lowland forests of southeastern Luzon and the smaller eastern islands of Polillo and Catanduanes (figure 1). Varanus mabitang is a poorly known species, endemic to Panay Island (Gaulke & Curio 2001). Both species are considered heavily threatened, primarily due to habitat destruction.

Because large species are conspicuous components of terrestrial faunas, they are usually well known to people. Here we describe a surprising finding from recent biodiversity studies: a giant (2 m body length), strikingly distinct, brightly coloured monitor lizard from the unexplored forests of northern Luzon Island.

2. Material and methods

Data were scored from fluid-preserved specimens deposited in US and Philippine collections and measurements were taken following Auffenberg (1988) and Gaulke & Curio (2001). We sequenced the mitochondrial 16S–ND1 fragments and nuclear PRLR, DNAH3, SNCAIP genes and incorporated these data into a previously published Varanus phylogenetic dataset. We subjected the combined data to phylogenetic analyses using parsimony, maximum likelihood and Bayesian methods (figure 1). The description of the new species is based on diagnostic characters of morphology. Full details of analyses and results are included in the electronic supplementary material.

3. Systematics

Varanus bitatawa sp. nov.

(a) Etymology

The specific epithet is derived from bitatawa, the Agta tribespeoples' common name for the new species.

(d) Brief description of holotype

A large-bodied species of Varanus, holotype snout–vent length (SVL) 766 mm, tail 1036; head robust, length 124.9, width 73, maximum depth 66.8, depth at eye 45.4; snout length 65.7; snout rounded anteriorly; narial openings slit-like, surrounded by an elevated protuberance; cranial table squarish, wider than long, with hypertrophied adductor musculature; head scales heterogeneous; supralabials 58; infralabials 66, decreasing in size to rictus; nuchals large, polygonal, decreasing in size laterally; scales arranged in 56 semi-regular rows from the posterior margin of the cranial table to forelimb insertion; scales of the dorsal trunk smaller than those on the head, polygonal; scales in 94 rows in the axilla–groin region; paravertebrals from the gular fold to the anterior edge of hindlimb insertion 127; axilla–groin distance 272 mm; limb scales large, polygonal, slightly convex, decreasing in size distally; forelimb and hindlimb 164 and 198 mm respectively; digits terminating in robust, recurved claws; scales of manus and pes squarish, smaller than those of trunk; supradigitals ovular; caudals rectangular, in semi-regular rows; scales around the base of the tail 101, around the tail one-third from the base, 59; double keel of paired, raised rectangular scales originating 110 mm from the base and extending posteriorly to tail terminus; ventrals less variable than dorsals; ventral nuchals small, increasing in size and becoming squarish through the mid-nuchal region, then decreasing in size anterior to the gular fold; scales around the neck anterior to the gular fold 145; gulars in 68 rows between the gular fold and the margin of the tympanum; ventrals posterior to the gular fold heterogeneous, larger than nuchals; scales in the scapular region polygonal, increasing in size posteriorly; midbody scales 193; ventrals from the gular fold to the anterior margin of hindlimb insertion 107; total ventrals 227; precloacals irregular, round to polygonal, decreasing in size distally through limbs; subcaudals small, heterogeneous, increasing in size posterior to the hemipenal bulge. Dorsal ground coloration black, accentuated with bright golden yellow in life; dorsum covered with golden yellow spots and flecks; head speckled yellow on black; forelimbs more yellow than black; trunk traversed by four distinct rows of yellow oscelli; hindlimbs black with large, distinct, yellow spots; tail barred, contrasting black and yellow. Detailed description of the type series is provided in the electronic supplementary material.

4. Discussion

Discovery of a highly distinctive new species of monitor lizard from heavily populated and highly deforested Luzon Island comes as an unprecedented surprise. How could such a large-bodied (2 m total length) monitor lizard have escaped the notice of the many biologists that have worked in the northern Philippines? We suspect a combination of factors have contributed to this astonishing set of circumstances. Despite being identified as a conservation priority (Mallari & Jensen 1993; van Weerd et al. 2004), surprisingly few surveys have characterized the herpetological diversity of the forests of Sierra Madre (but see Brown et al. 2000a, 2007). Additionally, if the new species is similar in habitat preferences and behaviour to its closest relative (V. olivaceus), V. bitatawa is most likely a highly secretive species that may never leave forests to traverse open areas (D. Bennett 2009, unpublished data). Despite escaping recognition by biologists, the new species is well known to resident Agta and Ilongot tribespeople who rely on it as a major source of protein (M. R. Duya & E. L. B. Rico 2004, unpublished data).

(a) Biogeography

The geographical gap between the new species and its sister taxon V. olivaceus, together with our new phylogenetic data, is key to our interpretation of the biogeographic and evolutionary history of these taxa. In hindsight, it is not surprising that the forested Sierra Madre Range supports an evolutionarily distinct lineage of monitor lizard. The northern and southern portions of the mountain range are geologically isolated (Auffenberg 1988; Hall 2002; Yumul et al. 2009) and are now separated by at least three non-forested river valleys representing barriers to dispersal (figure 1). This series of topological, ecological and atmospheric barriers to dispersal has resulted in a more than 150 km gap, isolating the northern portions of the range of V. olivaceus from the southern portions of the range of V. bitatawa (figure 1). Our hypothesized history of isolation of these new species is entirely consistent with (i) the allopatric distributions of V. bitatawa and V. olivaceus on either side of these three low-elevation valleys (figure 1), (ii) the high levels of genetic divergence and sister relationship between the two taxa (figure 1 and table 2), and (iii) the concomitant morphological differentiation between the two species (figure 2; electronic supplementary material). The discovery of the new species identifies a disjunct distribution pattern that may embody a more general, and under appreciated, biogeographic phenomenon affecting other, unrelated lineages. If low-elevation valleys are barriers to gene flow in montane forest species endemic to the separate geological components of Luzon, we might expect to see additional north–south differentiation in other lineages, sister species pairs or closely related higher taxa. Although available data are few, recent findings suggest that the Mid-Sierra Filter Zone detected here may have similarly affected other lineages (Brown et al. 2000a,b, 2007) and may therefore have a more general biogeographic significance.

(b) Conservation

Our unexpected finding of a highly conspicuous new species of a large vertebrate that has escaped discovery in the forests of the northern Luzon emphasizes the unexplored nature of the Philippines. The discovery of a new Varanus, an anticipated flagship species for conservation, adds to the recognition of the Philippines as a global conservation hotspot and a regional superpower of biodiversity (Brown & Diesmos 2009).